Method for encapsulating a chip on a carrier

ABSTRACT

Method for encapsulating a chip ( 10 ) on a carrier ( 12 ) whereby the chip ( 10 ) is positioned on the carrier ( 12 ), a dam ( 16   a   , 16   b ) is formed on the carrier ( 12 ) around the chip ( 10 ), encapsulating material ( 18 ) is poured into the space within the dam ( 16   a   , 16   b ), and the obtained encapsulated structure is cured. In a first separate step on the carrier only the dam ( 16   a   , 16   b ) is formed from a thermohardening material at a relatively high temperature and is cured at least partly and in a succeeding second step the space within the dam is filled with the same thermohardening material at a relatively low temperature.

BACKGROUND OF THE INVENTION

The invention relates to a method for encapsulating a chip on a carrierwhereby.

the chip is positioned on the carrier

a dam is formed on the carrier around the chip

encapsulating material is poured into the space within the dam

the obtained encapsulated structure is cured.

A prior art method of this type is described in JP-04302457. Accordingto this method the dam is a premanufactured separate framework which ispositioned on the carrier such that it surrounds the chip and is adheredto the carrier by means of an adhesive. Thereafter the encapsulatingmaterial is poured into the space within the dam. Premanufacturing,storing and handling of the frameworks is considered expensive andlaborious. The premanufacturing process and the encapsulating processare carried out separately and in different processing stations. Allthis is considered as a serious disadvantage of this method.

Also JP-61101054 obviously describes the use of a premanufactured damcomprising two sections, one sections made of a thermosetting resinmelting at high temperature and the second section made of athermosetting resin melting at low temperature. The second sectionfunctions as an adhesive element for positioning and fixing bothsections to the substrate after which the encapsulating material ispoured into the space within the dam. This method has the samedisadvantages as remarked in the preceding paragraph.

In JP 04303989 a method is described for reducing solder areas byforming a dam around the solder areas. On a substrate a lower conductorlayer is formed. Where necessary insulating material is applied andthereafter an upper conductor layer is formed. Together with the. upperconductors locally dams are formed on the lower conductors to mark outthe restricted soldering areas. The upper. conductors and the dams areconstituted as printed layers by applying a suitable printing paste andcuring the paste by backing. In a separate soldering step a component issoldered to the lower conductors whereby the amount of solder as well asthe soldering areas are delimited.

Another method is known from U.S. Pat. No. 4,961,886. In this knownmethod use is made of a material which will be cured under influence ofradiation, for instance ultraviolet radiation, electron beam radiation,or radiation in the visible part of the spectrum. By means of screeningmeans and focusing means care is taken that initially only a part of thecarrier is irradiated, i.e. the part where the dam has to be built.Thereafter the encapsulating material is poured onto the carrier at alocation within the dam. As soon as the material reaches the position ofthe dam it will be cured at least partly under the influence of thepresent radiation and in that way will form the dam. Within the daminitially the material is not cured. As soon as the dam is formed alsothe area within the dam is irradiated and the whole material is cured.

This prior art method has a number of disadvantages. In the first placethis method can only be. applied in combination with an encapsulatingmaterial which cures under the influence of radiation. Because of thatthe choice of the encapsulating material is relatively restricted. Afurther disadvantage relates to the fact that rather complicatedfocusing and screening measures are necessary to assure that theradiation from the radiation source will only irradiate that part of thecarrier where the dam has to be formed. A further disadvantage relatesto the fact that the pouring channel, through which channel theencapsulating material is poured onto the carrier, forms a hindrance forcorrectly radiating those parts of the carrier where the dam has to beformed. In U.S. Pat. No. 4,961,886 various solutions for this problemare indicated, all of them, however, necessitating additional measuresand making therefore the necessary apparatus more complicated.

A method showing a lot of resemblance with the one discussed in thepreceding two paragraphs is described in EP-0319175. Curable resinmaterial is poured onto a substrate. A predefined section of thesubstrate, more specifically the section were the chip is positioned, isirradiated by a suitable source and as soon as the poured resin materialreaches the irradiated section the material is cured and a dam is forcedaround the chip. Thereby the central part of the chip inside the dam isnot encapsulated to enable for instance chemical detection by means of achemically active part of the chip.

SUMMARY OF THE INVENTION

The object of the invention is now to indicate how the method defined inthe first paragraph for encapsulating a chip on a carrier using theformation of a dam can be realized cheaper and simpler.

In agreement with said object the invention now provides a method withthe characteristic

that in the first separate step on the carrier only the dam is formedfrom a thermohardening material at a relatively high temperature and iscured at least partly and

that in a succeeding second step the space within the dam is filled withthe same thermohardening material at a relatively low temperature.

By using a relatively high temperature in the first step thethermohardening material will rather quickly be cured, at least partly,and will form a dam of sufficient strength. In the second step the spacewithin the dam is filled at a relatively low temperature, so that thematerial within the dam at least initially will be fluid and willtherefore have the opportunity to flow out uniformly and fill the wholespace within the dam equally.

Preferably the method is carried out in such a manner that in a furtherthird step the obtained encapsulated structure, is cured at a thirdtemperature. Depending on the applied encapsulating material said thirdtemperature is preferably selected such that both the wall as well asthe material within the wall will be cured uniformly and preferablytensionless.

Although there are various methods to apply thermohardening material ata desired position on a carrier it is preferred that for the formationof the dam use is made of a first volumetric dispenser dispensing thethermohardening material. Suitable volumetric dispenser units are knownas such. Within the possible range of apparatuses it is preferred thatthe first volumetric dispenser comprises a dispensing mouth which can bemoved in two mutually perpendicular directions at some distance abovethe carrier such that a strip of material with the width and height ofthe dam can be dispensed.

With relation to the applied temperatures it is preferred that thetemperature during the first step is selected depending on the appliedthermohardening material such that the material, which is applied in theshape of a dam, after flowing out of the dispensing mouth shows no oronly a small amount of shape alteration. Preferably both the carrier aswell as the direct surroundings thereof have the desired temperature.

During the second step preferably both the carrier as well as the directsurroundings thereof have the desired relatively lower temperature.

In both steps it is furthermore preferred to take measures, ifnecessary, to prevent curing of the encapsulating material within thedispensing mouth of the dispensing unit. Such (cooling) measures are,however, known as such.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with references tothe attached drawings.

FIG. 1a shows the first of four successive stages in the anencapsulating process according to the invention wherein a chip ispositioned on a strip-type carrier.

FIG. 1b shows a second stage of the encapsulating process wherein a damhas been formed around the chip.

FIG. 1c shows a third stage of the encapsulating process wherein the damhas been filled with encapsulating material.

FIG. 1d shows a fourth stage of the encapsulating process wherein theencapsulated chip has been cured.

FIG. 2 shows an upper view on a carrier, as said carrier passes variousprocessing stations, in which the method steps according to theinvention are carried out.

FIG. 1 shows in four views 1 a . . . 1 d a number of succeeding methodsteps according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows a cross sectional view through a strip-type carrier 12onto which a chip 10 is positioned. Through a number of bonding wires,two of which indicated by 14 a and 14 b are visible in the figure, thechip is connected to the conducted pattern which is present onto or intothe carrier 12.

In a first method step a dam is built on the carrier 12 around chip 10.Therefore use is made of a so-called volumetric dispenser, dispensing astrip of material on the plate 12 of which the thickness is adjustable.The applied material is thermohardening and is applied at such arelatively high temperature of the carrier and the direct surroundingsthereof that the strip after leaving the dispensing mouth will curerelatively fast at least partly and will form on the plate 12 a dam ofwhich the dimensions will not significantly change anymore. In FIG. 1btwo parts of the dam are indicated by 16 a and 16 b. The chip 10 iscompletely surrounded by the dam. The dam may have a round, rectangular,or other shape. To be able to form a dam of the required shape it ispreferred that the first volumetric dispenser comprises a dispensingmouth which can be moved in two mutually perpendicular directions atsome distance above the carrier by means if which a strip of materialwith the width and height of the dam can be dispensed.

After applying the dam 16, again. using a volumetric dispenser, thespace within the dam is filled with a thermohardening material,preferably the same thermohardening material as was used for the dam.During this step, however, a lower temperature is applied so that thematerial to a large extent stays fluid and will fill the space withinthe dam uniformly such that the chip 10 and the wires 14 are completelyembedded. FIG. 1c illustrates a cross section through the situation atthe end of this step. During this step a volumetric dispenser can beapplied of which the dispensing mouth is at a fixed position.

The intermediate product obtained in this manner is conveyed through anoven in which a temperature is maintained such that both the dam 16 aswell as the filling material 18 therein are uniformly and tensionlesscured until finally a complete and in a sense tensionless uniformencapsulation 20 around the chip 10 and above the carrier 12 isrealized.

FIG. 2 illustrates how an elongated film, comprising a number of chips10, can be encapsulated in various processing stations within the scopeof the invention. The film-shaped carrier 12 is moving into thedirection of the arrow 14 (in the figure from left to right) along anumber of processing stations. Departing from a situation in which thechips in a non further elucidated manner are positioned on the carrier12, whereby the necessary bonding wires 14, which in FIG. 2 do not carryseparate reference numbers, are attached (bonded) to a conductorpatterns present in or on the carrier 12.

Within a zone 30 a first volumetric dispenser is installed for applyinga strip-shaped or rod-shaped dam onto the carrier 12. Suitabledispensers for dispensing a strip of material with suitable dimensionsand comprising a dispensing mouth which can be moved at least in twomutually perpendicular directions above the carrier 12 such that a damof the desired shaped can be dispensed on the carrier 12 are known assuch. As is indicated schematically in FIG. 2 within the zone 30 bymeans of such a disperser a dam 16 is applied around the chip 10. Thisdam may have in general a rectangular shape as indicated in FIG. 2, butalso other shapes such as a round or oval dam or a rectangular dam withchamfered edges are conceivable within the scope of the invention. Thetemperature within the zone 30 is maintained at a relatively high valuesuch that the material of the dam 16 will cure at least partly, in anycase to such an extent dat the dam as such will stay.

After further conveying the carrier 12 into the direction of the arrow40 the chip surrounded by the dam 16, will reach the zone 32, in which asecond dispenser is installed. By means of this second dispenser thespace within the dam 16 is filled with thermohardening material 18.Preferably the same material is used as the material in de dam 16. Thetemperature within the zone 32 is significantly lower than thetemperature within the zone 30 with the result that the material 18 willuniformly and equally flow and will fill the space within the dam 16completely. Therewith a good encapsulation of the chip 10 is realized.

The carrier 12 is conveyed further and the now encapsulated chip willreach the zone 34 in an oven 22 in which a temperature is maintainedsuch that during the passage of this oven the material 16, 18. is curedcompletely, uniformly and with any tension such, that after leaving theoven the encapsulation 20 on the carrier 12 has obtained its final curedstate.

Thereafter the separate encapsulated chips with corresponding section ofthe carrier can be separated from each other and processed any further,for instance installed on a credit card, etc.

What is claimed is:
 1. A method for encapsulating a chip on a carriercomprising: a) positioning the chip on the carrier; b) forming a damaround the chip on the carrier employing a volumetric dispenser, the dambeing formed from a thermosetting material at a first temperatureproviding at least partial curing of the thermosetting material; and c)subsequently to forming the dam, pouring a thermosetting material intothe space within the dam to encapsulate the chip, the thermosettingmaterial being at a second temperature being a temperature lower thanthe first temperature; wherein the volumetric dispenser comprises adispensing mouth positioned above the carrier and movable in twomutually perpendicular directions relative to the carrier to dispense astrip of material having the width and height of the dam and wherein theresulting encapsulated structure is cured to harden the encapsulatingmaterial.
 2. A method according to claim 1 wherein the thermosettingmaterial poured into the dam is the same material as is employed for damforming.
 3. A method according to claim 1 wherein the carrier ismaintained at said first temperature during dam forming.
 4. A methodaccording to claim 3 wherein the first temperature is such that the dammaterial cures quickly to form a dimensionally stable dam.
 5. A methodaccording to claim 1 wherein the carrier is maintained at said secondtemperature during chip encapsulation.
 6. A method according to claim 5wherein the second temperature is such that the thermosetting materialwill flow to uniformly fill the space within the dam.
 7. A methodaccording to claim 1 further comprising effecting the curing of theencapsulated structure at a third temperature.
 8. A method according toclaim 1 wherein the curing of the encapsulated structure is effected ata temperature selected according to the thermosetting material ormaterials employed, said curing temperature being maintained so that thedam and the encapsulating material are cured in a uniform andstress-free manner.
 9. A method according to claim 1 wherein thevolumetric dispenser is adjustable to adjust the thickness of thedispensed strip of material.
 10. A method according to claim 1comprising employing a further volumetric material dispenser to dispensethe chip encapsulation material, the further volumetric dispenser havinga fixed-position dispensing mouth.
 11. A method according to claim 1wherein during dam forming only the dam is formed and wherein the dam isfilled with the thermosetting material.
 12. A method according to claim1 wherein bonding wires connected to the chip are present within the damand are also encapsulated.
 13. A method according to claim 1 wherein thethermosetting material poured into the dam is the same material as isemployed for dam forming, wherein the carrier is maintained at saidfirst temperature during dam forming and at said second temperatureduring chip encapsulation, said first temperature being such that thedam material cures quickly to form a dimensionally stable dam and saidsecond temperature being such that the thermosetting material will flowto uniformly fill the space within the dam, and wherein the methodfurther comprises effecting the curing of the encapsulated structure ata third temperature to provide a uniformly cured stress-free product.14. A method according to claim 1 wherein the thermosetting materialpoured into the dam is the same material as is employed for dam forming,wherein the carrier is maintained at said first temperature during damforming and wherein the carrier is maintained at said second temperatureduring chip encapsulation.
 15. A method according to claim 1 wherein thechip is provided with bonding wires and wherein the dam is formed on thecarrier around the chip and bonding wires.